Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimating dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer. Normally the opposite side is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
The dye layer can be a monochrome dye layer or it may comprise sequential repeating areas of different colored dyes like e.g. of cyan, magenta, yellow and optionally black hue. When a dye-donor element containing three or more primary color dyes is used, a multicolor image can be obtained by sequentially performing the dye transfer process steps for each color.
Many of the dyes proposed for use in thermal dye sublimation transfer are not sufficient in performance because they yield inadequate transfer densities at reasonable coating coverages. Especially for transfer on transparant film materials as receiving element, the transfer densities obtained are too low.
The yellow mono-arylazoaniline dyes described in EP 432314 yield relatively high densities on transparant receiver elements. A disadvantage of these mono-arylazoaniline dyes is their high degree of retransfer, i.e. the re-transfer in course of time of part of the dye transferred to the receiving sheet from the transferred dye image to a sheet of paper or any other substrate in contact with the dye-receiving layer.
Therefore it is an object of the present invention to provide yellow dyes for use in thermal dye sublimation transfer materials which have high extinction coefficients and yield high transfer densities especially on transparant receiver elements and which show a decreased degree of retransfer.
A primary colored dye layer, for example, a magenta or cyan or yellow dye layer, may comprise only one primary colored dye (a magenta, cyan or yellow dye respectively) or may comprise a mixture of two or more primary colored dyes of the same hue (two magenta, two cyan or two yellow dyes respectively).
For obtaining black recording by thermal dye sublimation transfer, transfer is performed using a dye-donor element having a black colored layer usually containing a mixture of yellow, magenta and cyan colored image dyes. Mixtures of yellow, magenta and cyan dyes for the formation of a black colored layer are described in e.g. EP 453020, U.S. Pat. No. 4,816,435 and JP 01/136787.
Dye images transferred from dye-donor elements containing mixtures of dyes frequently show an increased fading rate due to a photochemical effect, known as catalytic fading of dye mixtures.
This phenomenon was investigated in textile dyeing by Rembold and Kramer (see Journal of the Society of Dyers and Colourists, vol. 94 (1978), pages 12-17) and by Asquith and Ingham (see Journal of the Society of Dyers and Colourists, vol. 89 (1973), pages 81-85). Catalytic fading is related to the observation that the light fastness of certain dyes applied to textiles alone is much better than when applied as mixtures. In most reported cases, the light fastness of cyan, violet or red dyes deteriorates when a yellow dye is added.
It is known that indoaniline dyes have a favourable effect on this phenomenon in that mixtures of dyes containing indoaniline dyes show less catalytic fading effects (see, e.g., the description in U.S. Pat. No. 4,933,226; U.S. Pat. No. 5,024,990; U.S. Pat. No. 5,026,679; EP 0 279 330; U.S. Pat. No. 4,816,435). Unfortunately these indoaniline dyes have poor light stability.. According to EP 423796 fading of a thermally transferred image can be obtained by including an anti-fading group in a dye. Amongst the numerous examples given is listed a bisanilino-azodye that has subsituted alkyls or its aniline nitrogens. It has however been found that transfer of this type of dyes is insatisfactory.
Therefore it is another object of the present invention to provide dye-donor elements, in particular black colored dye-donor elements, containing indoaniline dyes yielding transferred dye images showing no catalytic fading effects and improved light stability.